LED lamps, which utilize light-emitting diodes, are used in many displaying elements of instruments such as mobile devices, PC peripheral equipments, OA equipments, various kinds of switches, light sources for backlighting, and indicating boards. The LED lamps are strongly required not only to have high efficiency, but also to be excellent in color rendition when used for general lighting or to deliver a wide color gamut when used for backlighting. In order to increase the efficiency, it is necessary to adopt a highly efficient fluorescent material. Further, for improving the color rendition and for broadening the color gamut, it is effective to adopt a fluorescent material that emits green luminescence under the excitation with blue light.
On the other hand, high load LEDs generally become so hot while working that fluorescent materials used therein are heated up to a temperature of approx. 100 to 200° C. When the fluorescent materials are thus heated, their emission intensity is generally lowered. Accordingly, it is desired that the emission intensity be less lowered even if the fluorescent materials are heated.
In view of the above, a Eu-activated β-SiAlON phosphor can be regarded as an example of the fluorescent materials which emit green luminescence under the excitation with blue light and hence which are suitably used for the aforementioned LED lamps. That phosphor emits luminescence efficiently when excited at 450 nm, and its absorption ratio, internal quantum efficiency and esternal quantum efficiency under the excitation at 450 nm are approx. 65%, 53% and 35%, respectively.
There are proposed some SiAlON phosphors further improved in internal quantum efficiency and in esternal quantum efficiency. However, even those phosphors are still also wanted to be further improved in color purity.